Not Applicable
Not Applicable
Not Applicable
This invention relates in general to devices used for measuring the change in enthalpy of an air stream from preconditioned to conditioned states. In particular, this invention is a device for determining the rating of air conditioning or forced air heating units (hereinafter xe2x80x9cair-conditioning unitsxe2x80x9d), using the change in enthalpy between entry and exit air streams.
A class of devices for determining the rating of air conditioners and heaters is known in the art as enthalpy tunnels or code testers. The basic design of an enthalpy tunnel is described in the American Society of Heating, Refrigeration and Air Conditioning Engineers, (ASHRAE) Standard 37-1988, Methods of Testing for Rating Unitary Air-Conditioning and Heat Pump Equipment, the disclosure therein hereby incorporated by reference. Enthalpy tunnel tests measure wet and dry bulb temperatures of entry and exit air from air-conditioning units and determine the enthalpy of the air mass in both conditions. These measurements allow the difference in enthalpy between the entry and exit air masses to be calculated. The enthalpy difference, combined with the mass flow rate of the air is then used to determine the amount of work the air conditioner or heater has performed.
ASHRAE Standard 37-1988 provides the general requirements of an enthalpy tunnel and further provides tolerances and standards for associated instrumentation and ancillary equipment. However, the Standard provides only general guidelines for the structure of an enthalpy tunnel, not a specific design.
In general, enthalpy tunnel testing units consist of an indoor room containing the heating or cooling coil and the test equipment, and an outdoor room containing the compressor and diffusor unit, such an arrangement usually referred to as a test cell. The indoor and outdoor rooms are thermally separated; waste heat generated by the air-conditioning unit exhausted to the outside room, and ultimately to the atmosphere. The enthalpy tunnel is connected to the heating or cooling coil under test by means of duct-work, and a controlled, measured mass of air is directed across the coil. The enthalpy tunnel accomplishes three goals: (1) It controls and measures the flow rate of air through the heating or cooling coil; (2) It controls the static pressure drop across the heating or cooling coil, and (3) It measures the exit temperature and humidity of the air volume exiting the heating or cooling coil being tested.
The exit temperature and humidity combined with the exit volumetric airflow rate yields the exit mass flow and enthalpy of the conditioned air. The inlet temperature and humidity combined with the same mass flow rate for the exit yields the unit inlet enthalpy of the air. The difference between the two is the work performed by the unit. The same mass flow rate inlet and outlet, along with the energy calculations, is an expression of the First Law of Thermodynamics.
The guidelines given in ASHRAE Standard 37-1988 for the construction of an enthalpy tunnel are non-specific as to the physical structure of enthalpy tunnels but are suggestive. State of the art enthalpy tunnels follow this suggestion, and generally consist of a rectangular sheet metal tunnel with a series of diffusion screens and flow control devices. Current art enthalpy tunnels also use a bank of fixed nozzles for flow control, and static pressure control as suggested by the Standard.
In an enthalpy tunnel constructed in accordance with the ASHRAE Standard 37-1988, using the design suggested therein, conditioned air enters the tunnel and then passes through a diffusion screen to facilitate mixing and homogeneity of the air stream. The air then passes through a bank of fixed nozzles, used to create a pressure drop and, in enthalpy tunnels of current design, control volumetric flow rates and static pressure at the exit of the air conditioning unit under test. The pressure drop across the nozzle bank is measured by means of a draft-range differential pressure transmitter. The resulting information is used to determine a volumetric flow rate. The air stream is then passed through a second diffusion screen and then into a discharge chamber. The temperature and humidity of the exit air stream are measured by sampling the air at the entrance to or in the discharge chamber. FIG. 5 of ASHRAE Standard illustrates the suggested configuration of this type of enthalpy tunnel.
The sampled air is measured to determine wet and dry bulb temperatures. ASHRAE Standard 37-1988 requires that the air velocity over the wet bulb temperature measuring instrument be 1000 feet per minute (fpm.) Velocities above or below 1000 fpm require that the wet bulb measurements be corrected in accordance with ASHRAE Standard 41.1-1986, the disclosure therein hereby incorporated by reference. Current art enthalpy tunnels have no reliable method for insuring that this velocity is maintained. Consequently, a correction calculation is routinely performed, allowing more error to enter the calculated rating of the unit under test. Subsequent calculations using wet and dry bulb temperatures in conjunction with the volumetric flow rate, are used to calculate the mass flow rate and the work performed by the unit.
ASHRAE Standard 37-1988 also requires enthalpy tunnels to be equipped with a discharge fan to control the static discharge pressure of the air conditioning unit being tested, and for means to vary the capacity of the fan. Current art enthalpy tunnels use the fixed nozzle bank described above to control the amount of air that passes through the tunnel. Depending on the desired static discharge pressure required, nozzles are plugged or freed to restrict or increase flow, indirectly varying the discharge fan capacity, the flow through the tunnel and the static discharge pressure the unit under test sees.
This method of varying flow and controlling static pressure, although widely used, has undesirable effects on the air flow in the enthalpy tunnel, and on the accuracy of the measurements made using this method. The use of a nozzle bank, as suggested by the Standard, creates multiple jets in the downstream air mass. Flow rates could be set using any number of nozzle configurations, and nozzle configurations are not consistent from test to test. The velocity profile across the tunnel after the nozzle bank is extremely irregular. This irregularity is in part corrected by the use of a diffusion screen downstream of the nozzle bank, but testing has shown that the velocity profile is still far from flat even after passing through the downstream diffusion screen.
In general, the results of testing air conditioning units using this xe2x80x9cplugged nozzlexe2x80x9d method of varying air flow have been inconsistent, non-repeatable and inaccurate. Depending on what nozzles are plugged and due to the rectangular shape of the tunnels, unstable zones of re-circulation and varying pressure gradients are created. These unknown quantities cause errors in both flow and pressure readings, and result in non-homogenous temperature profiles within the air stream. As a result, the temperature of the air that is sampled is non-uniform, and results in erroneous calculations. In addition, the xe2x80x9cplugged nozzlexe2x80x9d method of setting volumetric flow rate and/or static pressure does not allow for modification of these parameters during a test run. Temperature and humidity changes can and do often occur during tests.
Measurements and tests conducted by the Applicants have shown that the air flow patterns downstream of fixed nozzle banks, are non-uniform even when no nozzles are plugged. The use of multiple nozzles creates downstream of the nozzle bank unstable laminar and turbulent flow regions. These regions are exaggerated and especially prevalent when nozzles are plugged to control the air flow rate. In addition to the non-uniform flow present in the main tunnel, the air sampling configurations presently being used produce uneven flow and pressure gradients within the sampling tubes themselves. Most current art enthalpy tunnels use a grid array of piping as a sample collection device. Usually the array is constructed using tubing or piping connected with tees and elbows. The tubing is perforated and the grid is attached to a sample fan or blower. The grid arrays generally have a series of holes drilled into the piping or tubing that are uniform in size and regular in placement. No effort is made to ensure that the volume of air sampled from every hole is the same, or that the sample as a whole is truly representative of the enthalpy conditions present in the air mass.
Inconsistent and non-repeatable results caused by the design flaws in the current in design of enthalpy tunnels have forced the industry to incorporate correction factors into the calculations using the data from current art enthalpy tunnels. The result of the inaccurate testing and calculations is that air conditioning units are placed into the stream of commerce without a truly accurate representation of their efficiency rating or capacity.
It is the current state of the art in enthalpy tunnels to operate them in test cells and in fixed locations. In general, test cells and enthalpy tunnels require that duct-work be routed from the unit under test to the tunnel, and often these routings take circuitous and tortuous paths. A tunnel design imparting some flexiblity in the location of the tunnel with respect to the unit being tested would further condition the air flow prior to its introduction into the tunnel.
An improved and accurate enthalpy tunnel is presented. The present invention overcomes limitations in the prior art enthalpy tunnels, whose designs do not attempt to condition and control the airflow patterns within the main tunnel and air sampling subsystem. The present invention replaces the ASHRAE Standard 37-1988 suggested configuration, which is widely used without modification by the industry, with a design that meets the requirements of the Standard and provides consistent, controlled repeatable results. The test results from this enthalpy tunnel may be used without xe2x80x9ccorrection factorsxe2x80x9d as a true and accurate measure of the capacity of a tested air-conditioning unit.
The present invention departs from the current art design of enthalpy tunnels by conditioning the air mass flow rate so as to present a homogenous volume immediately upon introduction into the enthalpy tunnel. The air volume velocity is slowed and the air volume is completely mixed in a settling chamber.
The present invention also replaces the rectangular design in use in the industry with circular geometry, using a cylindrical tunnel shape as opposed to the suggested rectangular shape. The circular geometry creates a flat, uniform velocity profile. The present invention improves the design of current art enthalpy tunnels by using a single nozzle instead of a bank of nozzles, presenting to the sampling mechanism a smooth, stable and uniform flow profile and circumventing the need for instrumentation calibration due to changes in nozzle configuration. The sampling method used in the prior art is replaced with a sampling tunnel that also conditions its air flow profile, leading to consistent sampling.
The present invention also provides means for controlling the air velocity and mass flow rate in both the main tunnel and the sampling tunnel in real time during the performance of a test, using a feedback loop controller for both the sampling and main tunnels, and by providing variable flow rate discharge mechanisms. These improvements over the current art generate predictable airflow, temperature and pressure profiles within the system, and consistent repeatable test results.
It is another improvement of the present invention over the prior art that only one diffusion screen need be used. Multiple screens are required where the fixed nozzle bank is used for flow control and determination because of the significant disruption of the air flow profile. The present invention provides a variable capacity discharge blower eliminating the need for plugged nozzle flow and control, and multiple diffusion screens.